Ferrofluids or magnetic colloids are liquids with magnetic properties in which ferromagnetic materials are colloidally suspended. Such ferrofluids or magnetic liquids must show a high degree of stability (gravitational and magnetic field) in order to perform well in various commercial devices and be responsive to external magnetic fields. Generally a stable magnetic colloid or ferrofluid in a high magnetic field gradient requires small ferromagnetic particles of generally less than 100 angstroms in diameter. The ferromagnetic particles are typically coated with one or several separate layers of surfactants to prevent agglomeration in any particular liquid carrier.
Ferrofluids are widely known and used, and typical ferrofluid compositions are described, for example, in U.S. Pat. No. 3,700,595, issued Oct. 24, 1972, wherein anionic surfactants, such as fatty acids, alcohols, amines or amids and other organic acids are employed as dispersing surface active agents; U.S. Pat. No. 3,764,504, issued Oct. 9, 1973, wherein aliphatic monocarboxylic acids are employed as dispersing agents; U.S. Pat. No. 4,208,294, issued June 17, 1980, wherein a water based magnetic liquid is produced by the employment of C.sub.10 to C.sub.15 aliphatic monocarboxylic acids as acid dispersing agents; and U.S. Pat. No. 4,430,239, issued Feb. 7, 1984, wherein a stable ferrofluid composition is provided employing a phosphoric acid ester of a long-chain alcohol as a surfactant.
Various processes have been described for preparing magnetic colloids and ferrofluids, such as described more particularly in U.S. Pat. No. 3,917,538, issued Nov. 4, 1975, which provides a process for preparing an irreversibly flocked magnetic particle through the use of different dispersing agents which includes a variety of monionic and anionic surfactants, such as various petroleum sulfonates as the anionic surfactants and wherein the ferrofluids are prepared employing a grinding or ball mill technique; U.S. Pat. No. 4,019,994, issued Apr. 26, 1977, which employs a petroleum sulfonate with an aqueous carrier; U.S. Pat. No. 4,356,098, issued Oct. 26, 1982, which describes ferrofluid compositions composed of silicone-oil carrier and a dispersing amount of an anionic surfactant which forms a chemical bond with the surface of the magnetic particles as a tail group compatible or soluble in the silicone-oil carrier; and U.S. Pat. No. 4,485,024, issued Nov. 27, 1984, wherein a ferrofluid is produced through controlling the pH of the aqueous suspension of the ferromagnetic particles of an organic solvent together with surface active agents, such as fatty carboxylic acids.
A properly stabilized ferrofluid composition typically undergoes practically no aging or separation, remains liquid in a magnetic field and after removing of the magnetic field shows no hysteresis. Such a stabilized ferrofluid exhibits stability by overcoming generally three principal attractive forces: van der Waals, interparticles-magnetic and gravitational forces. The average particle needed in a ferrofluid depends on the selection of the ferromagnetic materials and typically may range from 20 to 300 .ANG., for example 20 to 200 .ANG., and for use in a very high magnetic field gradient may range up to 100 .ANG. in diameter. Typically, the ferromagnetic particles must be covered by one or more layers of the selected surfactant in order to provide stability in an exterior magnetic field gradient. While there are many known ways to obtain small particles of the ferrites, cobalts, irons and other ferromagnetic particles, the type of surfactant and dispersing agent needed to stabilize these particular particles is an important aspect of the formation of stable ferrofluid compositions and the method of preparing such compositions.
The ferrofluid compositions have been used in a wide variety of commercial applications, such as for ferromagnetic seals, as dampening liquids in inertia dampers, as heat transfer liquids in the voice coil of loudspeakers, as bearing liquids, as ferrolubricants, for domain detection, for oil prospecting, and other applications.